parse error message in case of assignment to literal

[MarcoGorelli]

closes #509

[codecov-io]

Codecov Report

Merging #510 (b732dcf) into master (0208b08) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #510   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines          734       734           
=========================================
  Hits           734       734           

Impacted Files	Coverage Δ
nbqa/output_parser.py	100.00% <ø> (ø)

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (0208b08) to head (b732dcf).
Report is 215 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff            @@
##            master      #510   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines          734       734           
=========================================
  Hits           734       734           

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